Methods for enhancing capacitors having roughened features to increase charge-storage capacity

ABSTRACT

Structures and methods for making a semiconductor structure are discussed. The semiconductor structure includes a rough surface having protrusions formed from an undoped silicon film. If the semiconductor structure is a capacitor, the protrusions help to increase the capacitance of the capacitor. The semiconductor structure also includes a relatively smooth surface abutting the rough surface, wherein the relatively smooth surface is formed from a polycrystalline material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of my prior application Ser. No.09/692,897, filed Oct. 19, 2000, which is a divisional of U.S. Pat. No.6,225,157, issued on May 1, 2001, which is a divisional of U.S. Pat. No.6,150,706, issued on Nov. 21, 2000. Other related patents andapplications include U.S. Pat. No. 5,969,983, issued on Oct. 19, 1999,which is a divisional of U.S. Pat. No. 6,150,706; application Ser. No.09/353,576, filed on Jul. 15, 1999, which is a divisional of U.S. Pat.No. 6,150,706; and application Ser. No. 09/353,574, filed on Jul. 15,1999, which is a divisional of U.S. Pat. No. 6,150,706.

TECHNICAL FIELD

The invention relates generally to integrated circuits and morespecifically to an integrated circuit capacitor having a barrier layerthat forms at least a portion of at least one of the capacitorelectrodes, and to an improved barrier layer.

BACKGROUND OF THE INVENTION

To increase storage density (the ratio of storage capacity to die size)and data-access speed, dynamic-random-access-memory (DRAM) manufacturerscontinue to reduce the geometries of and otherwise improve thestructures and components that compose a DRAM circuit. One suchcomponent is the capacitor that is used as the storage element of a DRAMcell and one such structure is a diffusion barrier layer. Another suchcomponent is an antifuse, which often has a structure that is similar oridentical to that of a capacitor.

Unfortunately, leakage and depletion often prevent DRAM manufacturersfrom shrinking the size of a DRAM-cell capacitor from its present size.Generally, leakage denotes the discharge current that flows through thecapacitor dielectric when the capacitor is open-circuited, and thus is ameasure of how fast the charge on a capacitor will leak away. In acapacitor with semiconductor electrodes, e.g., polysilicon, depletiondenotes the affect of the depletion regions that form within theseelectrodes when the capacitor stores a charge. As the amount of leakageor depletion increases, the capacitor's storage capacity decreases. Butunfortunately, the storage capacity of a DRAM capacitor can be reducedonly so much before the DRAM cell can no longer hold its state betweenrefresh cycles, and thus can no longer store data reliably. Therefore,because the storage capacity of a capacitor is proportional to the areaof the capacitor plates, the area, and thus the overall size, of a DRAMcapacitor often must be relatively large to compensate for thestorage-capacity-robbing affects of leakage and depletion. That is, theDRAM capacitor often must be larger than it would have to be if leakageor depletion were reduced or eliminated.

Furthermore, conventional electrode material, such as polysilicon, oftencauses the access speed of a DRAM cell to be relatively slow. Often, theresistance of an electrode formed from such a material is relativelyhigh. Therefore, because this resistance is effectively in series withthe DRAM capacitor, it causes the time constant for charging/dischargingthe capacitor to be relatively large, and thus causes the DRAM cell tohave a relatively long read/write time.

Additionally, conventional barrier materials often prevent manufacturersfrom reducing the dimensions of a structure disposed in a barrier layer.A barrier layer is often used to prevent the dopant in one layer fromdiffusing into an adjacent layer during circuit processing. A popularbarrier material is tungsten silicide. But unfortunately, tungstensilicide crystallizes at about 800° C. and forms relatively largegrains. This crystallization degrades tungsten silicide's barrierproperties by orders of magnitude because dopants can easily diffusealong the grain boundaries. The large grains also prevent the use oftungsten silicide with relatively narrow structures such as wordlines.That is, if the structure's width is about the same as or is less thanthe grain size, tungsten silicide often cannot be used. Furthermore,although it can sometimes be used as such a barrier layer, titaniumnitride oxidizes easily, and thus is unsuitable for use in manyapplications.

Moreover, conventional electrode materials may cause a circuit coupledto an antifuse to have a relatively slow access speed. An antifuse has astructure similar to that of a capacitor, but is typically used as aone-time programmable, nonvolatile storage element. For example, anantifuse can be “blown” into a short-circuited state by applying aprogramming voltage that is high enough to break down the dielectricsuch that the electrodes contact each other through the dielectric.Unfortunately, the relatively high resistance of conventional electrodematerials may cause a blown antifuse to have a relatively highresistance. Because the circuit coupled to the antifuse often has aparasitic capacitance associated therewith, the relatively large timeconstant of the coupled antifuse electrodes and parasitic capacitancecan cause the circuit to have a relatively slow access speed.

SUMMARY OF THE INVENTION

An illustrative aspect of the present invention includes a semiconductorstructure with a rough surface having protrusions formed from an undopedsilicon film. If the semiconductor structure is a capacitor, theprotrusions help to increase the surface area of one of the electrodesof the capacitor, and hence, the capacitance of the capacitor. Thesemiconductor structure also includes a polycrystalline surface abuttingthe rough surface. The polycrystalline surface helps to enhance thestructural integrity of the semiconductor structure should perforationsexist in the rough surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a capacitor according to oneembodiment of the invention.

FIG. 2 is a cross-sectional view of a capacitor according to anotherembodiment of the invention.

FIG. 3 is a cross-sectional view of a transistor having a gate structurethat includes a barrier layer according to an embodiment of theinvention.

FIG. 4 is a schematic diagram of a DRAM cell that uses the capacitor ofFIG. 1 or FIG. 2 or the transistor of FIG. 3.

FIG. 5 is a block diagram of a memory circuit that can incorporate thecapacitors of FIGS. 1 and 2, the transistor of FIG. 3, or the DRAM cellof FIG. 4.

FIG. 6 is a block diagram of a computer system that incorporates thememory circuit of FIG. 5.

FIG. 7 is a cross-sectional view of a conventional semiconductorstructure.

FIG. 8 is a cross-sectional view of a semiconductor structure accordingto one embodiment of the present invention.

FIG. 9 is a cross-sectional view of a semiconductor structure accordingto one embodiment of the present invention.

FIGS. 10A-10G are cross-sectional views of a semiconductor structureundergoing an in-situ processing technique according to one embodimentof the present invention.

FIGS. 10H-10K are cross-sectional views of a semiconductor structureundergoing an ex-situ processing technique according to one embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a cross-sectional view of a DRAM capacitor 10 according to oneembodiment of the invention. The capacitor 10 includes a conventionalelectrode 12, which is formed from a conductive material such aspolysilicon. The electrode 12 is adjacent to one side of a conventionaldielectric 14, which is formed from an insulator such as silicondioxide, barium strontium titanate, or tantalum pentaoxide. Anotherelectrode 16 is adjacent to another side of the dielectric 14, and iscoupled to a DRAM-cell access device, such as a transistor. Theelectrode 16 is formed from a barrier material, and thus may be called abarrier electrode. The barrier electrode 16 may include conventionalbarrier materials, such as titanium nitride, or may include tungstennitride, tungsten silicon nitride, or titanium silicon nitride, whichare discussed below in conjunction with FIG. 3. Furthermore, althoughdescribed for use in a DRAM cell, the capacitor 10 can be used for otherapplications and in other integrated circuits such as microprocessors.

The barrier electrode 16 allows the capacitor 10 to be significantlysmaller than a conventional capacitor. For example, due to itsrelatively high work function, the barrier electrode 16 increases theheight of the barrier to electron flow through the dielectric 14, andthus reduces the leakage of the capacitor 10. Furthermore, because thebarrier electrode 16 is not a semiconductor material like polysilicon,there is no depletion in the electrode 16. Therefore, the overalldepletion associated with the capacitor 10 is significantly reduced.Also, in one embodiment, the barrier electrode 16 is thinner than aconventional electrode such as the electrode 12. For example, aconventional polysilicon electrode such as the electrode 12 may be 600 Åthick, but the barrier layer 16 may be as thin as 10 to 100 Å.Additionally, the barrier electrode 16 has a significantly lowerresistance than conventional electrodes such as the electrode 12, andthus reduces the series resistance of the capacitor 10. This alsoreduces the time constant associated with the capacitor 10, and thusincreases the access speed of the DRAM cell that includes the capacitor10. Moreover, the barrier electrode 16 also prevents dopants fromdiffusing from another layer into the dielectric 14, and preventsdopants in the dielectric 14 from diffusing out into other layers.

In another embodiment of the capacitor 10, the electrode 12 can beformed from silicon germanium, which has a lower work function thanpolysilicon. Thus, such an electrode 12 presents a higher barrier to theelectron flow through the dielectric 14 than does polysilicon, and thusfurther reduces the leakage of the capacitor 10.

In still another embodiment, the electrodes 12 and 16 can be reversed.That is, the electrode 16 can be formed from a conventional electrodematerial such as polysilicon or from silicon germanium as discussedabove, and the electrode 12 can be the barrier electrode. Alternatively,both the electrodes 12 and 16 can be barrier electrodes made from thesame or different barrier materials. In such an embodiment, the leakageand thickness of the capacitor 10 are further reduced, and the depletionis virtually eliminated.

Furthermore, although shown having planar sides in FIG. 1 for clarity,in an embodiment where the electrode 12 is a barrier electrode, theelectrode 16 may be formed with a rough or bumpy surface to increase itssurface area. One conventional material that is suitable to form such arough electrode 16 is hemispherical silicon grain (HSG) polysilicon.Also, the dielectric 14 and electrode 12 are formed such that theyconform to the adjacent rough surface of the electrode 16, and thus alsohave increased surface areas. Therefore, the increased surface areas ofthe electrodes 12 and 13 and the dielectric 14 increase the capacitanceof the capacitor 10.

In yet another embodiment, the capacitor 10 can be used as an antifuse.As previously discussed, an antifuse is a programmable, nonvolatiledevice that is normally electrically open but can be programmed tobecome electrically closed, i.e., a short circuit. For example,referring to the capacitor 10, to form one or more short circuits 18, asufficient voltage is applied across the electrodes 12 and 16 so as tocause the dielectric 14 to break down and the short circuit 18 todevelop between the electrodes 12 and 16. Because the electrode 16 is abarrier electrode, the series resistance of such an antifuse issignificantly reduced as is the time constant associated with theantifuse. Therefore, circuitry coupled to the antifuse can operate at ahigher speed than with a conventional, higher-resistance antifuse.

FIG. 2 is a cross-sectional view of a DRAM capacitor 20 according toanother embodiment of the invention. The capacitor 20 includes a firstelectrode 22, which may be one continuous layer or may include separatelayers 24 a and 24 b. A first barrier layer 26 is disposed adjacent toone side of the electrode 22, a conventional dielectric 28 is disposedadjacent to an opposite side of the barrier layer 26, a second barrierlayer 30 is disposed adjacent to another side of the dielectric 26, anda second electrode 32 is adjacent to an opposite side of the barrierlayer 30. Like the first electrode 22, the second electrode 32 may beone continuous layer or may include separate layers 34 a and 34 b.Furthermore, like the barrier electrode 16 of FIG. 1, the barrier layers26 and 30 may be formed from conventional barrier materials such as atitanium nitride, or may be formed from tungsten nitride, tungstensilicon nitride, or titanium silicon nitride. The barrier layers 26 and30, however, can be formed such that they do not form a silicide withthe adjacent electrodes 22 and 32, respectively. Alternatively, one orboth of the layers 24 b and 34 a may be silicide layers, or respectivesilicide layers may be disposed between the layers 24 b and 34 a and thedielectric 28. Additionally, although described as used in a DRAM, thecapacitor 20 may be used in other applications as well.

As discussed above in conjunction with the barrier electrode 16 of FIG.1, the barrier layers 26 and 30 reduce the leakage, depletion, seriesresistance, and thickness of the capacitor 20 as compared withconventional capacitors, and thus allow the capacitor 20 to besignificantly smaller than conventional capacitors. For example, byincreasing the barrier to electron flow through both sides of thedielectric 28, the barrier layers 26 and 30 significantly reduce theleakage of the capacitor 20. Furthermore, where one or both of theelectrodes 22 and 32 are formed from polysilicon or anothersemiconductor material, the respective barrier layers 26 and 30 (havingrespective thicknesses as low as 10 to 100 Å in one embodiment) allowthe respective thicknesses of the electrodes 22 and 32 to be reducedfrom approximately 600 Å (the typical thickness of a conventionalsemiconductor electrode as discussed above in conjunction with FIG. 1)to approximately 200 Å. Thus, not only does this reduction in thicknessreduce the overall thickness of the capacitor 20, it also significantlyreduces or eliminates the depletion that occurs in the electrodes 22 and32. Additionally, by providing conductive paths along the sides of thesemiconductor electrodes 22 and 32, respectively, the barrier layers 26and 30 allow the charge carriers within the electrodes 22 and 32 to moreeasily travel from one location to another, and thus significantlyreduce the series resistance of the capacitor 20.

In another embodiment, one or both of the electrodes 22 and 32 may beformed from silicon germanium, which, as discussed above in conjunctionwith the capacitor 10 of FIG. 1, further reduces the leakage of thecapacitor 20. Alternatively, the layers 22 and 32 may each include alayer of silicon germanium and a layer of another conductive materialsuch as polysilicon. That is, for example, one of the layers 24 a and 24b may be formed from silicon germanium and the other layer 24 a and 24 bformed from a conductive material such as polysilicon. Likewise, one ofthe layers 34 a and 34 b may be formed from silicon germanium, and theother layer 34 a and 34 b formed from another conductive material suchas polysilicon.

Also, as discussed above in conjunction with FIG. 1, although shownhaving planar edges for clarity, in one embodiment, one or both of theelectrodes 22 and 32 are formed with rough edges to increase theirrespective surface areas, thus increasing the capacitance of thecapacitor 20. For example, the electrode 22, the electrode 32, or bothmay be formed entirely from HSG polysilicon, or one of the layers 24 aand 24 b of the electrode 22 or one of the layers and 34 a and 34 b ofthe electrode 32 may be formed from HSG polysilicon, and the other oneof the respective layers 24 a and 24 b, and 34 a and 34 b, may be formedfrom silicon germanium or another conductive material.

In yet another embodiment, the capacitor 20 can be used as an antifuseas discussed above for the capacitor 10 of FIG. 1.

FIG. 3 is a cross-sectional view of a transistor 40 according to anotherembodiment of the invention. The transistor 40 includes conventionalsource/drain regions 42 and 44, which are disposed in a substrate 46, achannel region 48, which is disposed in the substrate 46 between thesource/drain regions 42 and 44, a conventional gate insulator 50, and agate conductor 52, which includes a first conductive layer 54, aconductive barrier layer 56, and a second conductive layer 58. In oneembodiment, the conductive layers 54 and 58 are formed from conventionalmaterials. For example, the layer 54 may be a silicide layer and thelayer 58 may be a polysilicon layer.

The barrier layer 56 prevents a dopant from diffusing from the layer 54,through the layer 58, and into the layer 50 during processing of thetransistor 40. Where the layer 58 is a semiconductor material such aspolysilicon, such diffusion can degrade the gate oxide 50 by causingmobile trapped charges that change the characteristics of the transistor40, such as the threshold, and thus cause the transistor 40 to operateimproperly for its intended use.

Because, as discussed above, conventional barrier materials such astungsten silicide and titanium nitride are often not suited for thesmaller geometries of today's denser integrated circuits, the barrierlayer 56 is formed from tungsten nitride, tungsten silicon nitride, ortitanium silicon nitride. These materials provide many advantages overconventional barrier materials. For example, the silicon component oftungsten silicon nitride and titanium silicon nitride allows transistorformation using a conventional “no spacer” process flow, which includesfewer steps and thus is cheaper to implement than other types of processflows. Furthermore, the silicon component of tungsten silicon nitrideand titanium silicon nitride also increases the step coverage of thesebarrier materials as published by P. M. Smith et al., Chemical VaporDeposition of Titanium-Silicon-Nitride Films, Applied Physics Letter 70(23), American Institute of Physics, Jun. 9, 1997, pp. 3116-118.Although important at any geometry, step coverage becomes more importantas the geometries shrink in size. Additionally, tungsten nitride,tungsten silicon nitride, and titanium silicon nitride are morecompatible with conventional polysilicon electrodes, word lines, andinterconnects than are tungsten silicide and titanium nitride. Moreover,because tungsten nitride, tungsten silicon nitride, and titanium siliconnitride have relatively high crystallization temperatures, they retaintheir barrier properties even after thermal cycling.

FIG. 4 is a schematic diagram of a conventional DRAM cell 60, whichincludes a capacitor 62 and an access transistor 64. In one embodiment,the capacitor 42 has the same structure as either the capacitor 10 ofFIG. 1 or the capacitor 20 of FIG. 2 and the transistor 64 isconventional. In another embodiment, the capacitor 62 is conventionaland the transistor 64 has the same structure as the transistor 40 ofFIG. 3. In yet another embodiment, the capacitor 42 has the samestructure as either the capacitor 10 of FIG. 1 or the capacitor 20 ofFIG. 2, and the transistor 64 has the same structure as the transistor40 of FIG. 3. The access transistor 64 has an access terminal 66 coupledto a digit line 68, a gate 70 coupled to a word line 72, and a storageterminal 74 coupled to a data terminal 76 of the capacitor 62. Areference terminal 78 of the capacitor 42 is coupled to a conventionalcell plate (not shown) that is biased at a cell-plate voltage VCP. Inone embodiment, the capacitor plate that composes the reference terminal78 is actually integral with the cell plate. That is, the cell plateacts as the respective terminals/plates 78 for all of the capacitors 62coupled thereto. Typically, VCP is half of the supply voltage thatpowers a circuit that includes the cell 60.

FIG. 5 is a block diagram of a memory circuit 80, which can include thecapacitor 10 or the capacitor 20 of FIGS. 1 and 2, respectively, thetransistor 40 of FIG. 3, the DRAM cell 60 of FIG. 4, or a combination orsubcombination of these components.

The memory circuit 80 includes an address register 82, which receives anaddress from an ADDRESS bus. A control logic circuit 84 receives a clock(CLK) signal, and receives clock enable (CKE), chip select (CS), rowaddress strobe (RAS), column address strobe (CAS), and write enable (WE)signals from a COMMAND bus, and generates control signals forcontrolling the operation of the memory device 80. A row addressmultiplexer 86 receives the address signal from the address register 82and provides the row address to row-address latch-and-decode circuits 88a and 88 b for one of two memory banks 90 a and 90 b, respectively. Thememory banks 90 a and 90 b each include a large number of DRAM cells 60(FIG. 4) using one or more of several embodiments of the invention, asexplained above. During read and write cycles, the row-addresslatch-and-decode circuits 88 a and 88 b activate the word lines of theaddressed rows of memory cells in the memory banks 90 a and 90 b,respectively. Read/write circuits 92 a and 92 b read data from theaddressed memory cells in the memory banks 90 a and 90 b, respectively,during a read cycle, and write data to the addressed memory cells duringa write cycle. A column-address latch-and-decode circuit 94 receives theaddress from the address register 82 and provides the column address ofthe selected memory cells to the read/write circuits 92 a and 92 b. Forclarity, the address register 82, the row-address multiplexer 86, therow-address latch-and-decode circuits 88 a and 88 b, and thecolumn-address latch-and-decode circuit 94 can be collectively referredto as an address decoder.

A data input/output (I/O) circuit 96 includes a plurality of inputbuffers 98. During a write cycle, the buffers 98 receive and store datafrom the DATA bus, and the read/write circuits 92 a and 92 b provide thestored data to the memory banks 90 a and 90 b, respectively. The dataI/O circuit 96 also includes a plurality of output drivers 100. During aread cycle, the read/write circuits 92 a and 92 b provide data from thememory banks 90 a and 90 b, respectively, to the drivers 100, which inturn provide this data to the DATA bus.

A refresh counter 102 stores the address of the row of memory cells tobe refreshed either during a conventional auto-refresh mode orself-refresh mode. After the row is refreshed, a refresh controller 104updates the address in the refresh counter 102, typically by eitherincrementing or decrementing the contents of the refresh counter 102 byone. Although shown separately, the refresh controller 104 may be partof the control logic 84 in other embodiments of the memory device 80.

The memory device 80 may also include an optional charge pump 106, whichsteps up the power-supply voltage VDD to a voltage VDDP. In oneembodiment, the pump 106 generates VDDP approximately 1-1.5 V higherthan VDD. The memory circuit 80 may also use VDDP to conventionallyoverdrive selected internal transistors.

FIG. 6 is a block diagram of an electronic system 110, such as acomputer system, which incorporates the memory circuit 80 of FIG. 5. Thesystem 110 includes computer circuitry 112 for performing computerfunctions, such as executing software to perform desired calculationsand tasks. The circuitry 112 typically includes a processor 114 and thememory circuit 80, which is coupled to the processor 114. One or moreinput devices 116, such as a keyboard or a mouse, are coupled to thecomputer circuitry 112 and allow an operator (not shown) to manuallyinput data thereto. One or more output devices 118 are coupled to thecomputer circuitry 112 to provide to the operator data generated by thecomputer circuitry 112. Examples of such output devices 118 include aprinter and a video display unit. One or more data-storage devices 120are coupled to the computer circuitry 112 to store data on or retrievedata from external storage media (not shown). Examples of the storagedevices 120 and the corresponding storage media include drives thataccept hard and floppy disks, tape cassettes, and compact disk read-onlymemories (CD-ROMs). Typically, the computer circuitry 112 includesaddress data and command buses and a clock line that are respectivelycoupled to the ADDRESS, DATA, and COMMAND buses, and the CLK line of thememory device 80.

FIG. 7 is a cross-sectional view of a conventional semiconductorstructure 700 including a memory cell 701, which comprises a transistor703 electrically coupled to a capacitor 705 via a plug 718. Thetransistor 703 is built on a substrate 702 and is isolated from othertransistors (not shown) by a field oxide layer 704. The transistor 703includes highly doped areas 706 that act as source or drain regions, andalso includes a gate stack 707, which controls the flow of chargecarriers through a chanel region 709 defined between the highly dopedareas 706. The gate stack 707 includes a gate oxide layer 708, apolycrystalline silicon gate layer 710, a silicide layer 712, a gate caplayer 714, and spacers 716.

A nonconductive layer 720 electrically isolates the transistor 703 andstructurally supports the capacitor 705, and non-conductive layer 722surrounds the capacitor 705 to electrically isolate it from othersemiconductor devices (not shown). The capacitor 705 comprises asubstrate 724, which may be made from polysilicon-germanium; ahemispherical silicon grain (HSG) layer 726 is formed adjoining thesubstrate 724, and may be formed from a germanium-doped amorphoussilicon layer. The substrate 724 together with the HSG layer 726 forms abottom electrode 727 of the capacitor 705. A dielectric layer 728 isformed conforming to the HSG layer 726, and a top electrode layer 730 isformed on the dielectric layer 728.

As previously discussed with reference to FIGS. 1 and 2, the HSG layer726 increases the surface areas of the bottom electrode 727 and topelectrode layer 730 of capacitor 705, and hence, increases itscapacitance. The significance of this increased surface areas by the HSGlayer 726 may be understood by revisiting the physics of a capacitor inthe context of the trend of ongoing miniaturization of semiconductordevices: A measure of the ability of the capacitor to store charge iscalled capacitance. The capacitance is given by εA/d, where ε is thepermittivity of the dielectric, A is the area of the electrodes, and dis the distance separating the electrodes. Both the area A and thedistance d of the capacitor define the physical dimensions of thecapacitor, and as the area A increases the capacitance, the ability ofthe capacitor to store charge also increases.

Capacitors of memory cells are built in limited space so as to comportwith the ongoing trend of miniaturization of semiconductor devices. Evenwith such physical constraints, the capacitance of these capacitors mustbe kept high to store sufficient charge and avoid excessive dissipationof charge over time. One technique to raise the capacitance is toincrease the area A by roughening the surface of an electrode of thecapacitor. This roughening process forms hemispherical protrusions (orgrains) that protrude from the surface of the electrode. The curvaturesurface of each grain provides a greater area than the flat surface ofthe electrode from which each grain protrudes. Thus, the combinedsurface areas of the grains increase the area A of the electrode andthereby raise the capacitance of the capacitor.

The resulting roughened surface is also known as an HSG (hemisphericalsilicon grain) layer. While the HSG layer helps to increase capacitance,a further increase in capacitance may be desired as the space in whichthe capacitor is formed continues shrinking. Moreover, during formationof the HSG layer 726, atoms in a previously deposited layer are consumedto form the hemispherical protrusions and thereby form the HSG layer, aswill be understood by those skilled in the art. Atoms in the underlyingelectrode layer 724 may also be consumed during formation of the HSGlayer 726. Unwanted holes in the HSG layer 726 and electrode layer 724may result as such atoms are consumed from each layer, and such holescan allow underlying regions to be damaged during subsequent processingsteps. This limits the ability to form large hemispherical protrusionson the layer 726 to increase the area of the layer.

FIG. 8 is a cross-sectional view of a semiconductor structure 800according to one embodiment of the present invention. The semiconductorstructure 800 includes a memory cell 801A, which comprises a transistor803A coupled through the corresponding plug 718 to a capacitor 805A, anda memory cell 801B, which comprises a transistor 803B coupled throughthe corresponding plug 718 to a capacitor 805B. Some of the structuraldetails and components of the memory cells 801A and 801B are similar tothose discussed with reference to FIG. 7, and for the sake of brevity,such components have been given the identical reference numerals ascorresponding components in FIG. 7. These components and structuraldetails will not again be discussed in detail.

In FIG. 8, the capacitor 805A includes a first polycrystalline electrodelayer 824 formed to contact the corresponding plug 718 and an HSG layer826 formed on an inner surface 825 of the first polycrystallineelectrode layer. The first polycrystalline electrode layer 824 and HSGlayer 826 together form a bottom electrode 827 of the capacitor 805A.The first polycrystalline electrode layer 824 prevents damage tounderlying regions if openings or perforations in the HSG layer 826occur during formation of that layer, as will be described in moredetail below. In one embodiment, the HSG layer is formed from an undopedamorphous silicon layer, as will be discussed in more detail below. Inthe structure 800, some of the nonconducting layer 722 is removed toexpose an outer surface 829 of the electrode layer 824, and the HSGlayer 826 is also formed on the outer surface 829. A dielectric layer828 is then formed on the HSG layer 826 and on an upper portion 831 ofthe electrode layer 824. A top electrode layer 830 is formed on thedielectrode layer 828. Each of the layers 826-830 may be formed from thesame materials as the corresponding layers in the structure of FIG. 7 orother suitable materials as will be understood by those skilled in theart. The capacitor 805B has the same structure as that just describedfor the capacitor 805A, and thus, for the sake of brevity, will not bedescribed in more detail. The capacitors 805A and or 805B have increasedrespective capacitance values due to the increased areas of the bottomelectrode 827 and top electrode layer 830. The use of both the inner andouter surfaces of the first polycrystalline electrode layer 824 alongwith the HSG layer 826 deposited thereon increases the surface area ofthe bottom electrode 827 which, in turn, increases the surface areas ofthe dielectric layer 828 and top electrode layer 830. Thus, thecapacitors 805A, 805B have increased capacitance values relative to theconventional capacitors 705 due to the increased areas of the bottomelectrode 827 and top electrode layer 830.

FIG. 9 is a cross-sectional view of a semiconductor structure 900according to another embodiment of the present invention. Thesemiconductor structure 900 includes a memory cell 901A, including atransistor 903A coupled to a capacitor 905A, and a memory cell 901Bincluding a transistor 903B coupled to a capacitor 905B. Some of thestructural details and components of the memory cells 901A and 901B aresimilar to those discussed in FIG. 8, and for the sake of brevity, suchcomponents have been given identical reference numerals, ascorresponding components in FIG. 8. These components and structuraldetails will not again be discussed in detail.

The capacitors 905A and 905B are identical, and thus only the capacitor905A will be discussed in more detail. In the embodiment of FIG. 9, thecapacitor 905A once again includes the first electrode layer 824 formedto contact the corresponding plug 718 and an HSG layer 926 formed on theinner surface 825 of the first electrode layer. A portion of thenonconductive layer 722 is again removed to expose an outer surface 829of the first polycrystalline electrode layer 824. A dielectric layer 928is then formed at the outer surface 829 and on the upper portion 831 ofthe first polycrystalline electrode layer 824, and is also formed on theHSG layer 926. A top electrode layer 930 is then formed on thedielectric layer 928. In the capacitor 905A, the first polycrystallineelectrode layer 824 and HSG layer 926 form a bottom electrode 927 of thecapacitor. The bottom electrode 927 and top electrode layer 930 haveincreased areas relative to the bottom electrode 727 and top electrodelayer 730 in the conventional capacitor 705 of FIG. 7. These increasedareas are due to the area added by the use of the outer surface 829 ofthe first electrode layer 824. By omitting the formation of the HSGlayer 926 on the outer surface 829 of the layer 824, the structure ofthe capacitor 905A provides an increased capacitance while having arelatively small width W, which allows the capacitors 905A, 905B to bemore densely formed, as will be appreciated by those skilled in the art.

FIGS. 10A-10K are cross-sectional views of the semiconductor structure900 during processing according to one embodiment of the presentinvention. FIGS. 10A-10G illustrate the formation of the HSG layer 926via an in-situ processing technique while FIGS. 10H-10K illustrate theformation of the HSG layer via an ex-situ processing technique. Thediscussion in FIGS. 10A-10K illustrates a few of the steps associatedwith a sample fabrication process. The entire fabrication process is notdiscussed so as to focus on the embodiments of the present invention,and one skilled in the art will understand such overall fabricationprocesses and appreciate various other methods of fabrication that maybe utilized in forming the structures 800 and 900. One skilled in theart will also appreciate various fabrication processes that may beutilized informing the capacitor 805A, 805B and the structure 800 ofFIG. 8. For the sake of clarity, many of the reference numbers in FIGS.10A-10K, once discussed, may be eliminated from subsequent drawings.

In the in-situ process of FIGS 10A-10G, FIG. 10A is a cross-sectionalview of the semiconductor structure 900 during processing andillustrates that the nonconductive layer 722 is etched to form openings1000A and 1000B, each opening defining a container, which will house acorresponding capacitor 905A, 905B. The other semiconductor elements inFIG. 10A, such as the transistors 703 and plugs 718, are formed usingconventional techniques, and thus their formation will not be describedin detail. Briefly, the gate oxide 708 is grown over the channel region707 of the substrate 702, which can be formed from any suitablesubstance, such as lightly doped n-type or p-type material and a lightlydoped epitaxial layer on a heavily doped substrate. The field oxidelayer 704 may be deposited, patterned, and etched on the substrate 702and the polycrystalline silicon gate 710 formed by depositing apolycrystalline silicon layer over the gate oxide layer 708 and thenphotolithographed and etched appropriately. Impurities of theappropriate kind are implanted or otherwise introduced into thesubstrate 702 to form the highly doped source and drain regions 706. Thesilicide layer 712 is formed on the polycrystalline silicon gate 710 tocreate a metal/semiconductor junction. The gate cap layer 714 andspacers 716 are formed by depositing a dielectric layer which is thenphotolithographed and etched. The nonconductive layer 720 is formed overthese components and the plugs 718 are formed in the nonconductive layerto electrically contact the highly doped areas 706. The nonconductivelayer 722, such as borophosphorus silicate glass (BPSG), is depositedover the nonconductive layer 720.

FIG. 10B is a cross-sectional view of the semiconductor structure 900during the next sequence of in-situ processing in which a conductivelayer 1002 is deposited over the nonconductive layer 722 and into theopenings 1000A and 1000B. The conductive layer 1002 can be of anysuitable material that will form a polycrystalline structure so that themajority of atoms are sufficiently bound to resist being drawn out ofthe layer 1002 and contributing to the formation of the HSG layer 926,as will be discussed in more detail hereinbelow. This conductive layer1002 will be part of the bottom electrode 927 of the two capacitors905A, 905B, housed in the openings 1000A and 1000B, respectively. Onesuitable material for the layer 1002 includes a silicon-germanium alloy.The deposition process of the silicon-germanium alloy includeslow-pressure chemical-vapor deposition, which forms thesilicon-germanium alloy to a thickness of less than about 500 angstroms.In such a deposition process, at a temperature greater than about 500degrees Celsius, silane gas (Si_(n)H_(2n+2)) is allowed to flow alongwith phosphine gas (PH₃) and digermanium hexahydride (Ge₂H₆) orgermanium tetrahydride (GeH₄) on the surface of the nonconductivematerial 722 including the openings 1000A and 1000B. Thesilicon-germanium alloy will become polycrystalline at about 500 degreesCelsius during this sequence of processing. Because of thetransformation the layer 1002 from the silicon-germanium alloy to apolycrystalline structure, the conductive layer 1002 is also designated824, which is the reference number used to refer to this polycrystallineelectrode layer in FIGS. 8 and 9.

FIG. 10C is a cross-sectional view of the semiconductor structure 900during the next sequence of in-situ processing. After the transformationof the silicon-germanium alloy 1002 into the polycrystalline electrodelayer 824, silane gas is again allowed to flow to deposit an undopedamorphous silicon layer 1004 at an appropriate temperature. In oneembodiment, the temperature is less than about 550 degrees Celsius, inanother embodiment the temperature is less than about 450 degreesCelsius, and in yet another embodiment the temperature is about 300degrees Celsius. This deposition process forms the undoped amorphoussilicon layer 1004 at a thickness less than about 500 angstroms.

FIG. 10D is a cross-sectional view of the semiconductor structure 900during the next sequence of in-situ processing in which the undopedamorphous silicon layer 1004 undergoes a seeding process to form anumber of seeds 1006 on the surface of the undoped amorphous siliconlayer 1004. The seeding process begins by bathing the surface of theundoped amorphous silicon layer 1004 in silane gas at a flow rategreater than about 10 standard cubic centimeters per minute and lessthan about 30 standard cubic centimeters per minute. The temperatureshould be raised to greater than about 550 degrees Celsius and less thanabout 600 degrees Celsius. The seeding process is a precipitation ofsolids from a gaseous matrix to form the seed 1006. Nucleation is thefirst step of the seeding process, and it describes the clustering ofsilicon atoms on the surface of the undoped amorphous silicon layer 1004to randomly produce many nuclei or seeds 1006. Those seed 1006 that arelarger than a certain size are stable and as a result can participate inthe growing process, which will be described in more detail below.

FIG. 10E is a cross-sectional view of the semiconductor structure 900during the next sequence of in-situ processing in which thesemiconductor structure 900 undergoes an annealing process, which causesthe seeds 1006 (FIG. 10D) to grow into hemispherical protrusions andthereby transform the layer 1004 and seed 1006 into the HSG layer 926.The annealing process is at a temperature that allows silicon atoms fromthe undoped amorphous silicon layer 1004, which are within the vicinityof the seeds 1006, to be drawn into the seeds 1006. As more and moresilicon atoms are drawn into the seeds 1006, the seeds 1006 begin togrow and forms the hemispherical protrusions on the HSG layer 926 asdiscussed above. The annealing process preferably occurs for about 30minutes at a temperature greater than about 550 degrees Celsius and lessthan about 600 degrees Celsius.

Depending on the thickness of the undoped amorphous silicon layer 1004,too many of the atoms from the undoped amorphous silicon layer 1004 mayparticipate in the growing of the hemispherical protrusions, and thus,may lead to the formation of undesired perforations in the HSG layer926. The perforations may allow etching solutions, which are used insubsequent processing steps, to leak through and damage underlyingcomponents of the semiconductor structure 900. Because of the presenceof the polycrystalline layer 824, however, even if such perforations inthe HSG layer 926 occur, the polycrystalline layer 824 protects thesemiconductor structure 900 from any damage due to the leakage ofetchant solutions through the perforations. The atoms in thepolycrystalline layer 824, as explained above, are bound sufficiently inthe layer 824 so that most of the atoms will not be drawn into the HSGlayer 926 during the growing of the hemispherical protrusions on the HSGlayer 926. Thus, the polycrystalline layer 824 remains intact during theannealing process so as to provide a protective barrier should undesiredperforation of the HSG layer 926 occur.

FIG. 10F is a cross-sectional view of the semiconductor structure 900during the next sequence of in-situ processing in which thesemiconductor structure 900 as shown in FIG. 10E undergoes achemical-mechanical planarization process to remove portions of the HSGlayer 926 and layer 824. The planarization process removes the HSG layer926 except for the portions of that layer in the openings 1000A and1000B, and also removes most of the polycrystalline layer 824. Thestructure resulting after the chemical-mechanical planarization processis illustrated in FIG. 10F.

FIG. 10G is a cross-sectional view of the semiconductor structure 900during the next sequence of in-situ processing in which thesemiconductor structure 900 is photolithographed and etched so that mostof the nonconductive layer 722 is removed to expose the outer surfaces829 of the polycrystalline electrode layer 824 as shown. The etchingprocess includes an etch-back process, which uses an etching solutionformed from a 10:1 ratio of water to hydrofluoric acid. The remainingsteps to complete the formation of the capacitors 905A and 905B areconventional, and thus, for the sake of brevity, will not be describedin detail. For example, the deposition of the dielectric layer 928 (SeeFIG. 9) over the HSG layer 926 is followed by a deposition of aconductive material over the dielectric layer to form the top electrodelayer 930 to complete the formation of the capacitors 905A, 905B of FIG.9.

What has been discussed with reference to FIGS. 10A-10G involves theformation of the HSG layer 926 via an in-situ process. What will bediscussed below with reference to FIGS. 10H-10K illustrates theformation of the HSG layer 926 via an ex-situ process. The ex-situprocess begins similarly to the in-situ process with the formation ofthe openings 1000A and 1000B as discussed in FIG. 10A, which is followedby the formation of the conductive layer 1002 as discussed in FIG. 10B,and the formation of the undoped amorphous silicon layer 1004 asdiscussed in FIG. 10C. The similarity between the in-situ and ex-situprocesses ends there.

The ex-situ process of forming the HSG layer 926 is shown in FIGS.10H-10K. FIG. 10H is a cross-sectional view of the semiconductorstructure 900 during a sequenece of ex-situ processing in which thesemiconductor structure 900 of FIG. 10C undergoes a chemical-mechanicalplanarization process to remove the polycrystalline layer 824 and theundoped amorphous silicon layer 1004 except for the portions of theselayers in the openings 1000A and 1000B.

FIG. 10I is a cross-sectional view of the semiconductor structure 900during the next sequence of ex-situ processing in which thesemiconductor structure of FIG. 10H is photolithographed and etched toremove most of the nonconductive layer 722 and expose the outer surfaces829 of the polycrystalline electrode layer 824. The etching processincludes an etch-back process, which uses an etching solution formedfrom a 10:1 ratio of water to hydrofluoric acid.

The next step of the ex-situ process is shown in FIG. 10J, whichillustrates a seeding process being applied to form a number of seeds1006 on the surface of the undoped amorphous silicon layer 1004. Theundoped amorphous silicon layer 1004 is bathed in silane gas at a flowrate greater than about 15 standard cubic centimeters per minute. Thetemperature should be raised to greater than about 600 degrees Celsiusand less than about 650 degrees Celsius. Because the outer surfaces 829of the polycrystalline layer 824 are exposed, the seeds 1006 are formedon both the surfaces of the undoped amorphous silicon layer 1004 and onthe surfaces of the polycrystalline layers 824.

FIG. 10K shows the next sequence of the ex-situ process in which thesemiconductor structure 900 undergoes a high-vacuum annealing process soas to grow the seeds 1006 as shown in FIG. 10J into hemisphericalprotrusions and thereby form the HSG layers 926. In one embodiment, theannealing process occurs for about five minutes at a temperature greaterthan about 600 degrees Celsius and less than about 650 degrees Celsius.As discussed above, the atoms in the polycrystalline layer 824 aresufficiently bound in the layer 824 so that most of the atoms will notparticipate in the growing of the hemispherical protrusions. As aresult, the seeds 1006 on the surface of the polycrystalline layer 824are unlikely to grow significantly, and the surface of thepolycrystalline layer 824 remains relatively smooth. The polycrystallinelayer 824 once again acts as a protective barrier to protect regionsunderlying the layer 824 should undesired perforation of the HSG layer926 occur. The remaining steps to complete the formation of thecapacitors 905A and 905B are conventional, and thus will not bedescribed in detail. For example, the deposition of the dielectric layer928 (FIG. 9) over the HSG layer 926 is followed by a deposition of aconductive material over the dielectric layer to form as the topelectrode layer 930 and complete the formation of the capacitors 905A,905B of FIG. 9.

From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

1. A semiconductor structure, comprising: a polycrystalline layer; and arough layer formed from an undoped silicon, the rough layer being formedon the polycrystalline layer and including protrusions extending from asurface of the layer. 2-51. (canceled)